This invention relates to a circuit arrangement for temperature compensation of capacitive pressure sensors and differential pressure sensors, comprising two measuring capacitors, each of which is arranged in a respective feedback branch of two measuring integrators and whose capacitance valves, which vary as a function of the pressure or differential pressure to be detected, are converted into measurement signals by the measuring integrators the circuit also comprises a negative feedback circuit which generates a temperature-dependent activation signal for the measuring integrators from at least one measurement signal.
A circuit arrangement of this kind, which compensates for the temperature effect during the measurement of the differential pressure of a differential pressure sensor, is known from DE-PS 33 40 834. Therein, a respective circuit arrangement for temperature compensation is described for a two-chamber differential pressure sensor and a single-chamber differential pressure sensor. The two-chamber differential pressure sensor comprises two measuring diaphragms which enclose a space filled with an incompressible liquid. This space is subdivided into two parts by an electrically insulating partitioning diaphragm which is provided with layer electrodes on both sides. Opposite the layer electrodes further electrodes are arranged on the main body of the sensor in order to form two measuring capacitors. In response to a temperature variation, the dielectric constant of the liquid changes and hence also the capacitance of each measuring capacitor. In order to compensate for this temperature error, use is made of the sum of the measurement signals which depends on the temperature to the same extent as the difference between the measurement signals but which, however, are independent of the differential pressure. The differential pressure .DELTA.P can thus be determined by means of the following equation: ##EQU1##
The constant K1 describes the temperature-dependent zero shift and the constant K0 represents a proportionality constant. The circuit arrangement for temperature compensation in a two-chamber differential pressure sensor comprises two measuring integrators, the feedback branch of which includes a respective measuring capacitor. The capacitance values which vary as a function of the differential pressure to be detected are applied as measurement signals to a negative feedback circuit by the measuring integrators, which negative feedback circuit sums the measurement signals in order to form a control signal that is applied to a comparator for comparison with a reference value. The difference between the control signal and the reference value signal is applied to a controller which controls an oscillator which applies an activation signal to the measuring integrators. Control is realised so that the amplitudes of the measurement signals remain constant, also in the case of a variation of the capacitances of the measuring capacitors due to a temperature variation, as a result of a variation of the frequency and/or the amplitude of the activation signal. As a result, the differential pressure measurement value calculated from the activation signals by an arithmetic device also remains constant, despite a temperature variation of the liquid.
The single-chamber differential pressure sensor, however, consists of two electrically conductive diaphragms which enclose a cavity filled with a liquid. The diaphragms constitute a respective measuring capacitor, in conjunction with respective, oppositely situated layer electrodes provided on a main body. The temperature dependency of the difference between and the sum of the reciprocal capacitance values of the measuring capacitors is not the same. This dependency can be expressed by the following equations: EQU 1/C1+1/C2=e+f.DELTA.T (2) EQU 1/C1-1/C2=a+b T+(C+d.DELTA.T) .DELTA.P (3)
where a, b, c, d, e and f are constants, .DELTA.T is the difference between a reference temperature and the operating temperature, and .DELTA.P is the pressure difference. The determination of the constants a, b, c, d, e and f is diclosed in the cited document DE-PS 33 40 834. Using the equations (2) and (3), the following is obtained for the differential pressure: ##EQU2## where K2=b e-a f
K3=f-b PA0 K4=f+b PA0 K5=c f-d e PA0 K6=d
The circuit arrangement for temperature compensation in a single-chamber differential pressure sensor comprises not only the two measuring integrators but also an additional reference integrator, without which the effect of the temperature-dependent term c+d.DELTA.T cannot be eliminated. In the negative feedback circuit further arithmetical operations are performed, in addition to the summing of the two measurement signals, in order to generate the control signal to be compared with the reference value signal. The controller controls the oscillator supplying the activation signal for the measuring integrators and the reference integrator so that in the case of a temperature-imposed variation of the capacitance values of the measuring capacitors the amplitudes of the measurement signals remain constant under the influence of a variation of the frequency and/or the amplitude of the activation signal. The temperature-independent differential pressure is determined in a subsequent arithmetic device.
For forming a reference signal, the negative feedback circuit utilises a plurality of adders and multipliers. In order to form an activation signal it comprises a comparator, a controller and an oscillator.
For pressure sensors which do not operate on the basis of the differential pressure principle, the principle of the cited circuit arrangement (DE-PS 33 40 834) can also be used for temperature compensation. Such pressure sensors comprise at least one electrode which is arranged on a main body and whose counter electrode is provided on the diaphragm. In order to enable temperature compensation, in addition to the first measuring capacitor thus formed there is realised a further measuring capacitor which is constructed as a reference capacitor and which is formed by an electrode on the diaphragm and a counter-electrode on the main body. The measurement signals obtained by means of the measuring integrators can be applied to the negative feedback circuit, either individually or as a sum, for temperature compensation.